The United States has 169 active volcanoes. More than half of them could erupt explosively, sending ash up to 20,000 or 30,000 feet where commercial air traffic flies. USGS scientists are working to improve our understanding of volcano hazards to help protect communities and reduce the risks.Video Sections:-Volcanoes: Monitoring Volcanoes-Volcanoes: National Volcano Early Warning System-Volcanoes: Science for Public Safety